We propose to investigate the mechanisms that allow effective regulation of the mammalian circadian clock by a population of non-classical photoreceptor cells in the retina. These intrinsically photosensitive ganglion cells (ipRGCs) express a G-protein coupled receptor called melanopsin. Photon capture by melanopsin activates ipRGCs, which transmit action potentials directly to the master circadian clock in the suprachiasmatic nucleus, among other brain regions. IpRGCs are absolutely required for the clock to respond to light and melanopsin phototransduction plays an important role in this process. Our overarching hypothesis is that melanopsin phototransduction is tailored to circadian photoreception. More specifically, the circadian clock sums photons over broad intervals of time and space, diminishing the impact of local fluctuations in light intensity to obtin an accurate representation of the overall light level. The clock uses this information to synchronize itself to local time. However, the basis of spatiotemporal summation in circadian photoreception is not yet understood. Our goal is to dissect the ipRGC light response in this context. We will investigate the mechanisms that allow ipRGCs to signal continuously during illumination, which is necessary for temporal summation. In addition, we will explore how the subcellular organization of melanopsin phototransduction allows ipRGCs to respond to broad expanses of visual space for spatial summation. Our approach is to identify ipRGCs using a BAC-transgenic mouse that drives expression of a fluorescent protein under the melanopsin promoter, and employ a combination of in vitro electrophysiology and optical stimulation that allows analysis at the biophysical level. The significance of this work is that ipRGCs are the principal regulators of the clock, and the clock controls up to a fifth of all expressed genes in a given tissue. Dysregulation of the clock is implicated in cancer, obesity, mental illness, and othe ailments. Through a systematic investigation of ipRGC photoreception within the framework of circadian biology, our research has the potential to reveal mechanisms that maintain health and are compromised in disease.